I had the problem that one of the bolt hole threads, in the aluminum bracket was soo worn, that the bolt slowly started loosening itself.. almost to the point of falling out/off (Aluminum plate and steel bolt.. they say even on bicycles that that's a no no.. don't mix materials..)... it became noticeable through a high pitch squeal... i have now installed longer bolts with a nut on the inside (yes there is enough room, just used a thinner nut and a whole gob of threadlock/thread fastener..) so you might want to check the tightness of the bolts... not saying it has to be that,, but it COULD be....
Acc-Man